1. Technical Field
The present invention relates to devices that use two sets of rollers to convey sheet media to a processing position, and relates more particularly to a conveyance device, printing device, and conveyance method that can maintain a consistent amount of slack in the sheet medium between the two sets of rollers and eliminate the effect of back tension on the downstream rollers without increasing device size.
2. Related Art
A means of conveying paper or other types of sheet media is required in order to process such media in a printer or other device. Such conveyance devices generally have upstream rollers that supply the media from the part storing the sheet medium to the conveyance path, and downstream rollers that convey the supplied medium through the conveyance path to the position where the media is printed or otherwise processed.
Such conveyance devices must be able to accurately control the conveyance speed of the medium from the downstream rollers in order to apply the printing or other process to the conveyed medium with good precision and high quality. Such control is difficult, however, if there is back tension from the upstream side pulling on the medium at the downstream rollers.
Technology for overcoming this problem is taught in Japanese Unexamined Patent Appl. Pub. JP-A-2008-56367. JP-A-2008-56367 teaches advancing the drive time or increasing the paper feed distance of the upstream rollers.
However, consistently controlling media conveyance appropriately as conveyance conditions change is a problem with the technology taught in JP-A-2008-56367 because the drive time is advanced or the paper feed distance is increased in the same way regardless of the current conditions. For example, the force applied to the rollers and the conveyance force of the rollers change according to roller wear and media storage conditions (such as the roll diameter when the medium is roll paper). As a result, if control is always based on the same fixed values, slack in the medium between the upstream rollers and downstream rollers may be lost, or excess slack may allow the medium to rub against parts disposed along the conveyance path, and back tension may be applied to the downstream rollers. Another problem is that device size must be increased in order to maintain excess slack at all times so that slack is not eliminated and prevent slack media from touching parts along the conveyance path.
More particularly, when media conveyance continues for a long time, the conveyance distance of both rollers may also change even when conveying the medium at a constant speed due, for example, to how the conveyed medium is stored (roll paper slack, fanfold paper), exacerbating the problems described above.